Does Pot Really Lower IQ?

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Marijuana permanently lowers IQ by several points in adolescents,
according to research published in August. But a new study
suggests that factors related to economic class and home life,
not marijuana use, may have caused that IQ drop.

Researchers who conducted the new study, published today (Jan.
14) in the journal Proceedings of the National Academy of
Sciences, did not have access to the original data from a cohort
of New Zealand teenagers. Instead, Ole Rogeberg, a labor
economist at the Frisch Center in Norway, used a computer
simulation to show that, in theory, the same IQ effects could be
explained by socioeconomic factors that cause IQ-decreasing
behaviors.

The new results are sure to ignite a firestorm of debate over
whether marijuana
permanently harms the growing brain.

"The kinds of environments you are in do affect your IQ,"
Rogeberg said. Good education and challenging jobs can boost
intelligence, but "if people are pushed out or decide to move out
of these kinds of arenas, they will tend to see an IQ decline,
and they will also be the type of people who tend to take up
cannabis
smoking during adolescence."

The original study can't rule out the possibility that other
factors could play a role in the link between IQ and marijuana
usage, said Susan Tapert, a neuropsychologist at the University
of California, San Diego, who was not involved in the study, in
an email.

"This was an excellent study – large, and followed the
same individuals over time. Even still, the possibility of
some other unmeasured issue to drive this link remains," Tapert
wrote.

The initial study, by researchers Madeline Meier, Terrie Moffitt
and Avshalom Caspi of Duke University, relied on an unprecedented
amount of data from a cohort of 1,037 people from Dunedin, New
Zealand, who have been followed for more than three decades. The
exhaustive study tracked people from age 7 onward, subjecting
them to
IQ tests, collecting blood samples, and interviewing parents
and teachers at several points along the way.

But socioeconomic class and family structure can change
IQ over time. For instance, poorer children tend to choose
less challenging schools or work, even if they start out with the
same IQ as higher-income kids. Economic class may also affect the
development of traits like motivation and perseverance that could
affect later-life IQ.

Rogeberg created a mathematical model to see if the apparent
effect of marijuana on IQ could be explained by socioeconomic
factors.

He found they could.

That doesn't mean marijuana is harmless, Rogeberg told
LiveScience. Instead, the original researchers should do a more
thorough analysis to prove that marijuana, and not other aspects
of the pot-smoking lifestyle, cause lower IQ, he said.

That should include testing to see whether cannabis use altered
individual people's
IQ trajectories from age 7 onward. Research should also look
at the cumulative impact of various factors, such as home life or
education level, that may negatively impact IQ, to see if this
cumulative effect could explain the drop seemingly caused by pot
smoking.

"It might be the totality of factors could explain a large part
of the effect that they found," Rogeberg said.

In response, the authors of the past scientific paper conducted a
new analysis and found that IQ overall remained fairly stable in
the Dunedin participants, they said in a statement. They also
only included kids from middle-class families, eliminating the
effects of socioeconomic status, they said.

But Rogeberg says those steps can't rule out other factors as the
cause for lower IQ.

"While interesting, the new analysis is not at all sufficient to
address these concerns – but I look forward to seeing a fuller
analysis in time," Rogeberg said.